In recent years, in hygiene products such as disposable diapers, sanitary napkins and incontinent pads, the water-absorbing resin as a composition material thereof has been widely used, in view of absorbing body liquid. As such a water-absorbing resin, for example, there have been known a cross-linked product of a partially neutralized polyacrylic acid, a hydrolysate of a starch-acrylic acid graft polymer, a saponified product of a vinyl acetate-acrylate ester copolymer, a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer, or a cross-linked product thereof, and a cross-linked product of a cationic monomer and the like. This water-absorbing resin may be used after converted to also a sheet-like, fiber-like or film-like form, however, it is generally used in a water-absorbing agent after converted to a powder-like form (particulate). As such powders (particles), for example, the particulate water-absorbing agent, having a weight average particle diameter thereof of about 200 to 800 μm, is used widely.
The particulate water-absorbing agent is produced via many steps including a drying step, a crushing step, a classification step, a surface cross-linking step, and the like. In an industrial scale production, a transportation apparatus is used for conveyance between apparatuses performing each step. As a conveyance system, mechanical conveyance and pneumatic conveyance are included. In mechanical conveyance, for example, a conveyor is used as the transportation apparatus.
The pneumatic conveyance has many merits as compared with the mechanical conveyance. As merits of the pneumatic conveyance, less mechanical parts, less mechanical troubles, superiority in durability, a one way line without necessity of a return line which is different from a belt conveyer and the like, less commingling of foreign materials and the like are included.
A system of pneumatic conveyance is largely classified to low density pneumatic conveyance and high density pneumatic conveyance. In low density pneumatic conveyance, a powder substance to be conveyed is in a dispersed state in a transportation pipeline. In high density pneumatic conveyance, a powder substance to be conveyed moves inside the transportation pipeline while forming an aggregate called a slag or a plug. Because high air flow velocity is required for the powder substance to maintain a dispersed state, low density pneumatic conveyance is high-velocity conveyance. On the other hand, high density pneumatic conveyance is low-velocity conveyance.
During pneumatic conveyance, collision or friction may occur in particles themselves of a substance to be conveyed (powder substance). In addition, in pneumatic conveyance, the powder substance may collide with a pipeline. In addition, collision or friction may occur between the powder substance and the pipeline. The powder substance may be worn or broken during pneumatic conveyance. Pneumatic conveyance may give influence on property of the water-absorbing resin powder substance. Therefore, pneumatic conveyance which is capable of suppressing property decrease of the powder substance is required.
Non-Patent Document 1 discloses various pneumatic conveyance methods using secondary air. Patent Document 1 discloses a conveyance method for the water-absorbing resin powder substance containing a polyhydric alcohol, which is capable of suppressing property decrease. Patent Documents 2 and 3 disclose pneumatic conveyance methods using secondary air in combination. Patent Document 4 discloses a method for preventing deterioration of powder substance characteristics caused by particle fracture during conveyance.
In addition, other than the above Documents, conveyance methods for the water-absorbing resin powder substance have been proposed, and Patent Documents 5 to 7 disclose pneumatic conveyance methods for the water-absorbing resin powder substance, which provide suppressed property decrease during conveyance. Aiming at such an object, specifically, Patent Document 5 specifies curvature radius of a pipeline to equal to or higher than 5 times diameter of the pipeline, Patent Document 6 specifies gas speed and solid-gas ratio, and Patent Document 7 has specifies Froude number. Still more, Patent Document 8 discloses a method for preventing aggregation of the water-absorbing resin powder substance during conveyance by heating or keeping heated of an apparatus, and discloses pneumatic conveyance as one example of a conveyance method.